Aircraft icing is known to occur on the aircraft exterior surface when the aircraft is airborne as well as when the aircraft is stationary on the ground. Airbone aircraft icing generally occurs on the leading edges of the airfoil when the surface temperature is at or below freezing. Unfortunately, this can occur at any time of the year when there is moisture or precipitation and when the aircraft is at or above the freezing altitude. Such aircraft ice formation or accretion can have deleterious effects on flight performance: lift decreases, thrust falls off, drag and weight increase, and stall speed dramatically increases. In recent years, undetected airborne icing has contributed to a number of catastrophic crashes and continues to threaten general aviation and high performance jet aircraft.
In the prior art, several de-icing and anti-icing systems have been developed to combat airbone aircraft icing. Such systems are typically classified in accordance with the means used to prevent or remove the icing--either thermal, chemical or mechanical. Furthermore, de-icing and anti-icing systems differ in their method of operation.
At present, two types of thermal anti-icing systems are currently in general use. Each type heats ice prone surfaces to a temperature sufficient to prevent the formation of ice. The two basic thermal systems are the so-called "bleed-air" and electro-thermal systems. The former uses exhaust gas from the aircraft to heat air which is then fed into cavities behind ice prone areas so as to melt the ice or prevent it from forming. Although these anti-icing systems are reliable, they consume a great deal of energy in operation.
Electro-thermal systems use resistive elements embedded within thin sheets of material placed over and/or under the ice prone surfaces. Electricity is supplied from a power source and passes through resistive elements which generates heat sufficient to melt the ice. Such electro-thermal systems likewise are power intensive. Moreover, attempts to minimize the energy consumption by melting the ice periodically, rather than continuously, often leads to "runback re-freeze" a condition whereby water from the melting ice flows to adjacent areas on the airfoil and refreezes.
Other anti-icing systems have attempted to solve the disadvantages of thermal systems. For example, so-called "weeping wing" systems have been developed. A chemical, such as an antifreeze solution, is pumped through capillaries distributed over the ice prone surfaces. The antifreeze solution prevents the ice from forming. However, in order to function properly the system must be in operation prior to the aircraft encountering icing. This limitation coupled with the environmental concerns of introducing such chemicals into the environment reduces the desirability of such systems.
De-icing systems, unlike anti-icing systems which prevent the formation of ice, remove already existing ice accretions from the surface of the aircraft. They generally exist in the form of pneumatic or electro-magnetical impulse actuators. Pneumatic systems utilize inflatable rubber bladders, such that when they expand, ice is sheared, cracked, and flaked off. The actuator part of the device is installed as a thin cap (a boot) that covers the ice prone area. Rubber pneumatic de-icer boots are widely used, but are unfortunately prone to damage from weather and foreign objects--typically, having a service life of 2-5 years. Further, they cannot effectively remove thin layers of ice less than about 0.25." One alternative pneumatic system under development utilizes a high pressure pneumatic pulse that causes the icing surface to move with a high acceleration and low displacement. Although attractive, this type of system requires a considerable amount of auxiliary equipment and, as such, is expensive and rather bulky. Further, although such systems use erosion resistant polymers, its durability falls far short of the possibility of using metals.
Electromagnetic de-icing systems have also been widely investigated. See, for example, U.S. Pat. Nos. 4,690,353 and 5,129,598 which are incorporated herein by reference. They perform in substantially the same manner as the impulse pneumatic system, but use current induced magnetic effects that results in either surface vibrations or strong pulses to effect de-icing. For example, in U.S. Pat. No. 4,690,353, Haslim et al. describe an electro-expulsive separation system in which an elastomeric covering referred to as a blanket, cuff, or boot is placed on an aircraft surface. Mutually repelling conductors are employed that distend the blanket abruptly when a current is applied, thus propelling ice from the protected surface. More specifically, current discharged into the conductors gives rise to magnetic fields of like polarity that result in an electro-mechanical excitation. However, to obtain the required expulsion force, an extremely high current flow through the conductors is required. Such a current flow may be generated by capacitively storing and then discharging a high charge through the conductors. To effect discharge, very high current rated silicon control rectifiers must be used. Such high power rectifiers are, however, bulky and costly. Moreover, to achieve such high currents for actuators of a practical size, high voltages must be used, making the system susceptible to internal shorting. At modest current levels, the expulsion force affords only a limited de-icing capability. Also, like the pneumatic boot these systems generally require an auxiliary mechanism to eliminate "auto-inflation" that occurs when polymer boots having internal hollow voids are used to make the system more effective using modest current levels.
Alternative electro-magnetic de-icing systems use induced eddy currents which act on the surface of the leading edges of metal airfoils to effect de-icing. See, for example, U.S. Pat. Nos. 4,399,967 and 5,129,598, which are incorporated herein by reference. Although electro-magnetic de-icing systems are potentially capable of removing very thin layers of ice and their performance is generally superior to pneumatic de-icers, they generally suffer from some or all of the following problems: limited distribution of the de-icing effect, low power efficiency, bulk, and limited service life.
Accordingly, there exits a need for a low cost, low weight improved de-icing system having, among other things, modest voltage and current requirements.